1. Field of the Invention
The present invention relates to a pulse modulation circuit, and more particularly to a pulse modulation circuit used for communication or radar.
2. Description of the Related Art
Up to now, modulation circuits, which include a harmonic mixer, are used in a Pulse-Doppler radar apparatus for a vehicle or the like (for example, refer to JP 2000-338233 A).
In the pulse modulation circuit disclosed in JP 2000-338233 A, the harmonic mixer is equipped with a harmonic mixer local oscillation (LO) signal input terminal for inputting an LO wave, and a harmonic mixer IF signal input terminal for inputting the IF signal that is generated by the pulse signal generator as well as an RF signal output terminal for outputting the RF signal into which the inputted LO wave and IF signal are mixed.
The operation will be described. The harmonic mixer inputs the LO wave to the harmonic mixer LO signal input terminal, inputs the IF signal to the harmonic mixer IF signal input terminal and mixes those signals together to output a sum frequency component of the second harmonic wave of the LO wave and the IF signal to the RF signal output terminal 9. A relation of fRF=2fLO+fIF is satisfied assuming that fLO is the LO frequency, fIF is the IF frequency and fRF is the RF frequency.
In the case where the dc component is inputted to the harmonic mixer IF signal input terminal as the IF signal, when a supply voltage of dc is 0, the second harmonic wave component of the LO wave is not outputted to the RF signal output terminal because there is no IF signal that is mixed with the LO wave. In the case of a supply voltage V1 (an arbitrary voltage other than 0) of dc, the LO wave and the dc component are mixed together, to thereby satisfy fRF=2fLO because of fIF=0, and the second harmonic wave component of the LO wave is outputted to the RF signal output terminal.
In the case where a DC pulse that has been generated in the pulse signal generator is supplied to the harmonic mixer IF signal input terminal, and the LO wave is supplied from the harmonic mixer LO signal input terminal, the LO wave is mixed with the DC pulse that has been supplied to the harmonic mixer IF signal input terminal in the harmonic mixer, and a mixed wave of the second harmonic wave of the LO frequency which is a higher harmonic wave of an odd order and the DC pulse signal is outputted from the RF signal output terminal 9 as a modulated wave.
In the case of the conventional pulse modulation circuit as described above, the second harmonic component of the LO wave should not theoretically be outputted at the supply voltage at the off time of the pulse. However, at the actual supply voltage at the off time of the pulse, there arises such a problem that the on/off ratio of the output power in the switching operation is reduced due to noises of several tens mV which is close to 0 V.
In addition, there arises such a problem that the on/off ratio is deteriorated due to manufacture variations of elements (for example, diodes) which structure the harmonic mixer.